The invention relates to an improved synthesis of N,N-disubstituted-p-phenylenediamine, especially the dibutyl form (DBPDA). DBPDA is a useful reagent in colorometric and fluorescence assays.
U.S. Pat. No. 6,066,467, the contents of which are incorporated by reference, discloses the use of a particular N,N-disubstituted-p-phenylene diamine, DBPDA, in a fluorescence assay for hydrogen sulfide production from the action of homocysteinase. The assay, which is useful to measure homocysteine levels in bodily fluids, shows enhanced sensitivity over similar assays which utilize absorbance by color. Such assays are described in U.S. Pat. Nos. 5,998,191 and 5,985,540, also incorporated by reference. The colorometric reagent counterparts to DBPDA, in addition to this dibutyl derivative, can also be N,N-dipropyl-1,4-phenylenediamine or the corresponding diethyl or dimethyl derivative.
While the dibutyl derivative, DBPDA, is available commercially, it is not as inexpensive or obtainable in as great quantity as the lower alkyl counterparts. Accordingly, an improved synthesis for DBPDA would be of benefit.
A classic synthesis for this compound is described in Reilly, J. and Hickinbottom, W. J. in J. Chem.Soc. (London) (1918) pages 99-111. Two methods are described for the synthesis; of greater relevance to the present invention is the conversion of di-n-butylaniline to the para-nitroso compound with subsequent reduction to the desired product. It has now been found that the described method is unworkable as strictly described, as it is necessary to conduct certain steps of the reaction in an oxygen-free environment. In addition, an improved method for crystallization is described herein. These improvements are applicable to N,N-disubstituted-p-phenylene diamine (DSPDA) generally.
The invention is directed to an improvement over the literature-described synthesis of di-n-butyl-p-phenylenediamine and its salts, as well as DSPDA in general. The improved method, like that described in the prior art, employs the conversions shown in FIG. 1. Both the intermediate nitroso-compound and the phenylenediamine derivative that is the final product can be converted to the acid addition salts using standard techniques. According to the method of the present invention, certain steps in this reaction are conducted in an oxygen-free environment.
Thus, in one aspect, the invention is directed to an improved method to synthesize DSPDA, especially DBPDA, which method comprises treating p-nitroso disubstituted aniline with a reducing agent, and working up the product in the absence of oxygen. In a preferred embodiment, the reducing agent is zinc. Further, the production of the nitroso compound is preferably from treating disubstituted aniline starting material with nitrous acid.
In another aspect, the invention is directed to an improved method to crystallize DSPDA, especially DBPDA, which method comprises treating an absolute ethanol solution of the dihydrochloride with ether.